ampeg svt2 pro trouble

Hello, I'm asking you if someone can tell me what's happend to my svt2 pro. I buy this head two years ago, and my band ater two weeks, stop to exist. The head stay in my garage inside a closed case for 15 months, now I take it to play with another band, and when I put the head on, I hear an awesome CRACK! from the speaker, like a machine gun! the nois go progressively down and then remain a noise at high frequencies like the first crack, but at a little volume level. So I shut down the head, and then I restarted; if head is hot, the initial CRACK does not come, if head is cold, her come the thunder, and than continue the little problem during all the session. What you think about that?